The invention relates to three dimensional coordinate measuring machines (CMM), and in particular to the measurement of large objects with high accuracy. The need for this type of measurement can be found in many industries such as automotive, machinery, construction, robotics, aerospace and others.
The limitation of conventional coordinate measuring machines for large objects are mainly the cost of the machine and the need to bring the large object to the machine. This becomes less and less practical as the object size exceeds a few meters.
Prior art attempts to solve the problem consisted of connecting the measuring probe to a fixed reference point via the use of articulated arms, such as U.S. Pat. No. 4,606,696 or use of optical methods based on interferometry. An example of optical methods is U.S. Pat. No. 4,627,722 which has limited range and accuracy or U.S. Pat. Nos. 4,790,651, 4,457,625, 4,621,926 and 4,707,129 which offers high accuracy but requires continuous tracking. Since these four patents operate on a purely interferometric principle, any breaking of the beam path will cause the measurement reference to be lost. In other words, the systems described in these four patents are accurate but incremental systems. In order to overcome this limitation of incremental systems and create an absolute measuring system LEICA-KERN (Switzerland) uses a combination of theodolites to measure angles and compute the location of the target. This is a commercially available system, however it has a limited accuracy and is slow.
The problem of measuring points which are not within line-of-sight to the transceivers is addressed by U.S. Pat. No. 4,691,446, French Patent 2,547,916 and commercially available units such as the sonic digitizer made by SAC (Stratford, Conn.)
All these devices use a three point probe in which two of the points are within line-of-sight to the transceivers. Since the points are in line and the distances between the points are accurately known, the location of the hidden point can be computed from the locations of the visible points.
It is the object of this invention to have a high accuracy absolute measuring system capable of measuring large objects. It is another object to provide a measuring system covering a large volume, such as a large area on the production floor, and being able to measure each object in that volume at all stages of fabrication and assembly. It is a further object to provide such a measuring system at a low cost and a modular manner. These and other objects will become apparent after studying the following description in conjunction with the drawings.